Mal Heron

The hydrodynamics of a bleaching event: implications for management and monitoring

This chapter examines the hydrodynamic conditions that are present during a coral bleaching event. Meteorological and climate parameters and influences are discussed. The physics of mixing and its influence on the horizontal and vertical variations of sea temperature are examined. A specialized hydrodynamic model for Palau is then presented as a case study to demonstrate the utility of these models for understanding spatial variations during bleaching events. This case study along with the other sections of this chapter provide the foundation for concluding that hydrodynamic modeling can provide us with a relatively accurate glimpse of the spatial variation of thermal stress and, therefore, what future stress events may hold for corals. Although the timing of a coral bleaching event is unknown and cannot be predicted with current technology, the relative patterns of sea surface temperature during individual bleaching events can be predicted using current modeling techniques. However, improvements in our understanding of coral physiology and higher spatial-resolution climate models are necessary before the full potential of these predictions can be utilized in management decisions.

Structure and influence of tropical river plumes in the Great Barrier Reef: application and performance of an airborne sea surface salinity mapping system

Input of freshwater from rivers is a critical consideration in the study and management of coral and seagrass ecosystems in tropical regions. Low salinity water can transport natural and manmade river-borne contaminants into the sea, and can directly stress marine ecosystems that are adapted to higher salinity levels. An efficient method of mapping surface salinity distribution over large ocean areas is required to address such environmental issues. We describe here an investigation of the utility of airborne remote sensing of sea surface salinity using an L-band passive microwave radiometer. The study combined aircraft overflights of the scanning low frequency microwave radiometer (SLFMR) with shipboard and in situ instrument deployments to map surface and subsurface salinity distributions, respectively, in the Great Barrier Reef Lagoon. The goals of the investigation were (a) to assess the performance of the airborne salinity mapper; (b) to use the maps and in situ data to develop an integrated description of the structure and zone of influence of a river plume under prevailing monsoon weather conditions; and (c) to determine the extent to which the sea surface salinity distribution expressed the subsurface structure. The SLFMR was found to have sufficient precision ( 1 psu) and accuracy (∼3 psu) to provide a useful description of plumes emanating from estuaries of moderate discharge levels with a salinity range of 16 to 32 psu in the open sea. The aircraft surveys provided a means of rapidly assessing the spatial extent of the surface salinity distribution of the plume, while in situ data revealed subsurface structure detail and provided essential validation data for the SLFMR. The combined approach allowed us to efficiently determine the structure and zone of influence of the plume, and demonstrated the utility of sea surface salinity remote sensing for studying coastal circulation in tropical seas.

Flushing time of solutes and pollutants in the central Great Barrier Reef lagoon, Australia

The flushing time of the central Great Barrier Reef lagoon was determined by using salinity as a tracer and developing both an exchange model and a diffusion model of the shelf exchange processes. Modelling suggests that the cross-shelf diffusion coefficient is approximately constant for the outer half of the lagoon but decays rapidly closer to the coast. The typical outer-shelf diffusion coefficient is ~1400 m2 s–1, dropping to less than 100 m2 s–1 close to the coast. Flushing times are around 40 days for water close to the coast and 14 days for water in the offshore reef matrix.

A Comparison of Algorithms for Extracting Significant Wave Height from HF Radar Ocean Backscatter Spectra

Abstract A comparison is made between three different but related algorithms for the extraction of rms wave heights from high frequency ocean backscatter radar spectra. All three methods are based on the ratio of second- to first-order energies as developed by Barrick, and each was scaled so that the mean values of the radar analysis results and the corresponding wave buoy data were zero. The rms difference between the radar wave heights and those from the buoy was taken as a measure of fit, and the recommended algorithm had an rms difference value of 7 cm. Barrick’s algorithm (after scaling), which uses a weighted second-order energy integral, performed marginally better than the others. The condition requiring wind directions other than close to orthogonal to the radar beam is retained in the recommended algorithm but is not evaluated because of sparsity of data. The algorithm for extraction of rms wave heights is validated against the buoy data over rms wave height ranges from about 0.2 to 0.7 m.

Tsunami observations by coastal ocean radar

When tsunami waves propagate across the open ocean, they are steered by the Coriolis effect and refraction due to gentle gradients in the bathymetry on scales longer than the wavelength. When the wave encounters steep gradients at the edges of continental shelves and at the coast, the wave becomes nonlinear and conservation of momentum produces squirts of surface current at the head of submerged canyons and in coastal bays. High frequency (HF) coastal ocean radar is well conditioned to observe the surface current bursts at the edge of the continental shelf and give a warning of 40 minutes to 2 hours when the shelf is 50 to 200 km wide. The period of tsunami waves is invariant over changes in bathymetry and is in the range 2 to 30 minutes. Wavelengths for tsunamis (in 500 to 3000 m depth) are in the range 8.5 to over 200 km, and on a shelf where the depth is about 50 m (as in the Great Barrier Reef (GBR)) the wavelengths are in the range 2.5 to 30 km. In the use of HF radar technology, there is a trade‐off between the precision of surface current speed measurements and time resolution. It is shown that the phased array HF ocean surface radar being deployed in the GBR and operating in a routine way for mapping surface currents, can resolve surface current squirts from tsunamis in the wave period range 20 to 30 minutes and in the wavelength range greater than about 6 km. An advantage in signal‐to‐noise ratio can be obtained from the prior knowledge of the spatial pattern of the squirts at the edge of the continental shelf, and it is estimated that, with this analysis, the time resolution of the GBR radar may be reduced to about 2.5 minutes, which corresponds to a capability to detect tsunamis at the shelf edge in the period range 5 to 30 minutes. It is estimated that the lower limit of squirt velocity detection at the shelf edge would correspond to a tsunami with water elevation of about 2.5 cm in the open ocean. This means that the GBR HF radar is well conditioned for use as a monitor of small, as well as larger, tsunamis and has the potential to contribute to the understanding of tsunami genesis research.

Effective complex permittivity of a weakly ionized vegetation litter fire at microwave frequencies

Thermal ionization of alkali species emitted from thermally decomposing vegetative matter into the combustion zone of a fire makes the zone a weakly ionized gaseous medium. Collision between the medium electrons and neutral flame particles is a dominant form of particle interaction and incident microwave energy absorption process. Electromagnetic wave absorption properties of vegetation fire have implications for the safety of fire fighters during wildfire suppression where communication blackouts have been experienced. Propagation characteristics of electromagnetic waves in a vegetation fire could be deduced from its relative dielectric permittivity. In the experiment, a controlled fire burner was constructed where various dried natural vegetation could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyser to determine effective complex permittivity from scattering parameters. A controlled vegetation fire with a maximum flame temperature of 1050 K was set in the burner and X-band microwaves (8.0–9.6 GHz) were made to propagate through the flame. For the flame, at temperatures of 800 and 1015 K, imaginary and real components of effective complex dielectric permittivity were measured to range from 0.113 to 0.119 and from 0.898 to 0.903, respectively.

The use of HF radar surface currents for computing Lagrangian trajectories: Benefits and issues

Surface coastal currents mapped by a pair of high frequency ground-wave radars (HFR) have been used to predict Lagrangian trajectories in the proximity of Heron Island (Capricorn Bunker Group, Great Barrier Reef, Australia), and to compare with the current data measured by an Acoustic Doppler Current Profiler (ADCP) at three mooring stations. Overall the HRF and ADCP absolute current speeds showed a difference less than ±0.15 m s−1 for 68% of the observations. A good agreement between HFR (at a depth of 1.5 m) and ADCP (at a depth of 5.5 m) data were observed for the u-component (cross-shelf) which presented a stronger tidal signal, while a poor comparison was found for the v-component (north-south) more influenced by the south-easterly and northerly winds. The HFR allowed inclusion of not only the temporal, but also the spatial current variability in the tracking computation. This proved to be crucial because the Lagrangian trajectories were very sensitive to the starting position and time in the studied area, where the currents exhibit a large spatial variation imposed by tides, winds, large scale circulation and topography. One challenge in applying HFR data for Lagrangian tracking consists of estimating the missing values and including the effects of small scale fluctuations.

Measurement of Electrical Conductivity for a Biomass Fire

A controlled fire burner was constructed where various natural vegetation species could be used as fuel. The burner was equipped with thermocouples to measure fuel surface temperature and used as a cavity for microwaves with a laboratory quality 2-port vector network analyzer to determine electrical conductivity from S-parameters. Electrical conductivity for vegetation material flames is important for numerical prediction of flashover in high voltage power transmission faults research. Vegetation fires that burn under high voltage transmission lines reduce flashover voltage by increasing air electrical conductivity and temperature. Analyzer determined electrical conductivity ranged from 0.0058 - 0.0079 mho/m for a fire with a maximum temperature of 1240 K.

Effect of Wildfire-induced Thermal Bubble on Radio Communication

Horizontal roll vortex pairs are dynamical structures that transfer energy and emissions from wildfires into the atmosphere. The vortices form at the edges of an intense line wildfire and emulate two cylinders, which form two curvatures of a biconcave thermal lens. Wildfire plume provides a dielectric material for the dielectric lens, whose permittivity is influenced by the nature, quantity of constituents (e.g., potassium and graphitic carbon) and variation of temperature with height in the plume. The environment created by the plume is radio sub-refractive with an effect of spreading radio wave beams. A numerical experiment was carried out to quantify loss of Ultra High Frequency (UHF) radio signal intensity when high intensity wildfireinduced horizontal roll vortices intercept UHF propagation path. In the numerical experiment, a collimated radio wave beam was caused to propagate along fuel-fire interface of a very high intensity wildfire in which up to two roll vortex pairs are formed. Maximum temperature of the simulated wildfire was 1200 K. Flame potassium content was varied from 0.5–3.0%. At 3.0% potassium content, a vortex pair imposed a maximum radio ray divergence of 2.1 arcmins while two vortex pairs imposed a maximum divergence of 4.3 arcmins at the fuel-fire interface. The ray divergences caused maximum signal strength loss (in decibels (dB)) per unit path length of 0.154dBm−1 and 1.65dBm−1 respectively.

Spatial Averaging of HF Radar Data for Wave Measurement Applications

HF radar data are often collected for time periods that are optimized for current measurement applications where, in many cases, very high temporal resolution is needed. Previous work has demonstrated that this does not provide sufficient averaging for robust wave measurements to be made. It was shown that improvements could be made by averaging the radar data for longer time periods. HF radar provides measurements over space as well as in time, so there is also the possibility to average in space. However, the radar data are correlated in space because of the range and azimuth processing. The implications of this are discussed and estimates of the impact on the reduction in variance in the radar Doppler spectral estimates are obtained. Spatial inhomogeneities and temporal nonstationarity in the ocean wave field itself also need to be taken into account. It is suggested that temporal averaging over periods of up to one hour and spatial averaging over 9–25 nearest neighbors may be suitable, and these will be explored in later work.